There is a continuing effort in polymer chemistry to develop polymers that exhibit macro functionality or to develop new functional polymers that possess specific chemical reactivity. These developments would extend the level of control available to materials engineers in processing polymers and using polymers as building blocks in, or components for, subsequent material forming reactions, such as copolymerization, chain extension and crosslinking reactions, and interaction with substrates, including dispersed solids. To be commercially useful, these reactions should start from readily available, low cost monomers and produce materials which are reactive during separate operations or during fabrication, for example, by reaction injection molding, compounding or alloying, and other processes to form coatings, fibers, films, composite structures or bulk articles, with modifiable and controllable desirable properties. A significant economic hurdle that has to be overcome in this effort is to provide the benefits of controlled polymerization, resulting in greater control over the preparation of materials from such available low cost monomers, exhibiting both micro- and macro-functionality, in available commercial process equipment. These long term objectives have provided the backdrop, or driving force, for the continuing advances in controlled polymerization of radically (co)polymerizable monomers, disclosed by some of the present inventors in earlier applications, and provide the incentive to extend, simplify and make more robust the process known as atom transfer radical polymerization (ATRP).
The most evolved version of the classic ATRP reaction is described in U.S. patent application Ser. No. 09/018,554, the entire contents of which are hereby incorporated herein by reference. Methods for exercising control over many parameters in a catalytic process for the controlled polymerization of a wide range of free radically (co)polymerizable monomers have been described in publications authored or co-authored by Krzysztof Matyjaszewski and others. See for example, Wang, J. S. and Matyjaszewsk, K., J. Am. Chem. Soc., vol. 117, p. 5614 (1995); Wang, J. S. and Matyjaszewsk, K., Macromolecules, vol. 28, p. 7901 (1995); K. Matyjaszewski et al., Science, vol. 272, p. 866 (1996); K. Matyjaszewski et al., “Zerovalent Metals in Controlled/“living” Radical Polymerization,” Macromolecules, vol. 30, pp. 7348-7350 (1997); J. Xia and K. Matyjaszewski, “Controlled/“Living” Radical Polymerization. Homogenous Reverse Atom Transfer Radical Polymerization Using AIBN as the Initiator,” Macromolecules, vol. 30, pp. 7692-7696 (1997); U.S. Pat. Nos. 5,807,937 and 5,789,487, the contents of each of which are hereby incorporated herein by reference. The subtle interactions between the parameters have been further explored and implementation of the teachings disclosed in these publications has allowed the preparation of many inherently useful novel materials displaying control over functionality and topology, and production of novel tele-functional building blocks for further material forming reactions, resulting from application of the site specific functional and topological control attainable through this robust controlled polymerization process for free radically (co)polymerizable monomers.
The system or process employed to gain control over the polymerization of free radically (co)polymerizable monomers has been described in earlier applications as comprising the use of four components: (i) an initiator molecule, or polymerization originator molecule and (ii) a transition metal compound having (iii) an added or associated counterion and the transition metal compound complexed with (iv) a ligand(s). The initiator molecule, or polymerization originator molecule can be any molecule comprising one or more radically transferable atom(s) or group(s) capable of participating in a reversible redox reaction with the transition metal compound. The transition metal compound includes an added or associated counterion. So that all reactive oxidation states are soluble to some extent in the reaction medium, the transition metal is complexed with ligand(s). The components of the system are chosen to (co)polymerize the added monomers. See U.S. Pat. No. 5,763,548, the entire contents of which are hereby incorporated herein by reference.
In an embodiment known as “reverse” ATRP, the initiator molecule described above can be formed in-situ by reaction of a free radical with the redox conjugate of the transition metal compound. Other components of the polymerization system such as the choice of the radically transferable atom or group, counterion initially present on the transition metal, and optional solvent can influence the process. In addition, the functions of the components of the system can be combined in a single molecule. U.S. Pat. No. 5,807,937 provides as an example of a single molecule containing a combination of functions, a complex in which the counterion and ligand are in one molecule. The role of the deactivator, the “persistent radical,” or for ATRP, the transition metal redox conjugate, is also described in U.S. Pat. No. 5,807,937.
It is still often advantageous to think of the process prerequisites individually so that one can conceptually consider the conditions for control over every aspect of the process. For example, if one wishes to introduce site specific functionality into the resulting polymer one can either add an initiator, or originator molecule containing the desired functional group, or a masked functional group if the desired group can interact with the transition metal complex, or one can utilize the radically transferable atom or group which will be present at the active growing polymer chain end(s) to introduce the desired functionality to the product after polymerization is complete.
While not to be limited to the following description, the theory behind ATRP is that polymerization proceeds essentially by cleavage (and preferably essentially homolytic cleavage) of the radically transferable atom or group from the rest of the initiator molecule or, during the polymerization process the dormant polymer chain end, by a reversible redox reaction with a complexed transition metal catalyst, without any strong carbon-transition (C-Mt) bond formation between the active growing polymer chain end and the transition metal complex. Within this theory as the transition metal complex, in a lower active oxidation state, or in its activator state, activates the initiator or dormant polymer chain end by homolytically removing the radically transferable atom or group from the initiating molecule, or growing polymer chain end, in a reversible redox reaction, an active species is formed that allows other chemistry, essentially free radical based chemistry to be conducted. This is a reversible reaction. The transition metal complex in the higher oxidation state, the redox conjugate state or deactivator state, transfers a radically transferable atom or group to the active initiator molecule or growing chain end, thereby reforming the lower oxidation state transition metal complex. When free radical based chemistry occurs, a new molecule comprising a radically transferable atom or group is also formed. In prior publications, the catalytically active transition metal compound, which can be formed in situ or added as a preformed complex, has been described as containing a range of counterions. The counterion(s) may be the same as the radically transferable atom or group present on the initiator, for example a halide such as chlorine or bromine, or may be different radically transferable atoms or groups. An example of the latter counterion is a chloride counterion on the transition metal compound when the initiator first contains a bromine. Such a combination allows for efficient initiation of the polymerization followed by a controlled rate of polymerization, and has additionally been shown to be useful in certain crossover reactions, from one set of (co)monomers to a second set of (co)monomers, allowing efficient formation of block copolymers.
Presently, a wide variety of vinyl monomers can be (co)polymerized in a controlled or “living” manner by this ATRP technique with an increasing number of demonstrated transition metals, e.g. copper, iron, nickel, ruthenium and rhodium in conjunction with different ligands. Many ligands are available for each transition metal used in ATRP, but, despite this, ligands that are cheaper and better able to form catalytically active complexes with improved redox potentials are still desired. In addition, there is a continuing desire to identify catalyst complexes that are amenable to recycle or reuse by known chemical manufacturing processes.